In the field of CMOS image sensors, there is a continuing need for pixel circuits having fewer transistors in order to permit increased resolutions and/or reduced surface area.
One technique often used for reducing the number of transistors in each pixel circuit is to use common reset and read transistors for multiple photodiodes. The number of transistors in a pixel circuit is generally expressed as the number of transistors per photodiode, and the use of multiple photodiodes can permit the number of transistors per photodiode to be reduced to two or less.
Indeed, in such a pixel circuit, each photodiode is usually associated with a corresponding transistor forming a transfer gate, for transferring charge accumulated by the photodiode to a sense node. The pixel circuit further comprises a reset transistor for resetting the photodiodes, and a read transistor for selectively coupling the sense node to an output line via a source follower transistor. Reference is made to United States Patent Application Publication No. 2006/0208163 (incorporated by reference) which further proposes a pixel circuit without any read transistor.
However, there are difficulties in reducing the number of transistors in each pixel circuit without also reducing the speed at which the pixels of the image sensor can be read. There is a need in the art to at least partially address one or more problems in the prior art.